1. Field of the Invention
The present invention relates generally to the technology of treating electrical cables with fluids. In particular, the present invention is directed to method and apparatus for treating electrical cables with fluids effective to retard "electrochemical tree" formation. The method of the present invention involves introducing and supplying fluid to a cable including a stranded conductor surrounded by an insulation layer in such a manner that interruption of the current of electricity through the conductor of the cable is not required.
2. Discussion of Background and Material Information
The transmission and distribution of electrical power is frequently accomplished using cables composed of an interior stranded conductor material covered or otherwise encased by insulation material. Such cables are particularly useful in underground cable systems wherein the cables may be directly buried or installed through ductwork. Regardless of the method of installation of the cables, the cables are normally exposed to environmental moisture throughout their use.
The following patents relate to representative examples of cable systems which may be used for such purposes. U.S. Pat. No. 3,594,492 is directed to a high-voltage electrical cable enclosed within a pipe which may be wrapped with a sheath or tapes of magnetic low-loss material.
U.S. Pat. No. 4,397,807 is directed to a cable for cryogenic use which includes an inner conductor and a solid polymeric insulator adapted to accommodate a collapsible spacer between the inner conductor and the polymeric insulator to further space the insulator away from the conductor so as to permit a greater shrinkage of the insulator. The invention claimed in this patent is the method of treating the cable for use in low temperatures without introducing damaging stresses during cool down in the insulation.
U.S. Pat. No. 4,394,534 relates to the previously discussed patent, and is directed to a coaxial cryogenic electrical cable having a collapsible spacer positioned between the inner conductor and the conductor shield.
U.S. Pat. No. 4,446,095 is directed to a method for manufacturing heat shrinkable tubing sleeves for use in splicing electrical cables.
U.S. Pat. No. 4,519,759 relates to the previously mentioned patent, and is directed to an apparatus used for manufacturing long links of such heat-shrinkable sleeves for use in splicing electrical cables.
U.S. Pat. No. 4,533,789 is directed to a high voltage electrical power cable with thermal expansion accommodation which includes an insulation shield having thick polymeric insulation constructed to have a metallic layer radially spaced from the inner semi-conducting layer.
U.S. Pat. No. 4,256,921 is directed to a moisture resistant high-voltage cable which has a composite jacket with inner and outer parts such that a corrugated moisture barrier is sandwiched between and bonded to these parts to prevent both radial and longitudinal flow of moisture in the cable.
Typically, however, high-voltage cables for underground installation are composed of an insulation layer of polyolefin material, such as polyethylene, cross-linked polyethylene or ethylene propylene rubber. A problem associated with the polyolefin covering of such electrical cables is that over time environmental moisture, which has a high dielectric constant and high conductivity, diffuses throughout the insulation layer of the cable through the intermolecular spaces in the insulation, which are relatively large in polyolefin materials, and tends to accumulate therein. The accumulated moisture forms into droplets, both in the intermolecular spaces and in microvoids and voids which may be present in the body of the insulation. Under electric fields, electroforces act on the droplets of moisture causing them to elongate in the direction of the field thereby decreasing the radii of the ends of the droplets which effects an increase in the strength of the electric field at the narrowed ends of the droplets. At sufficiently high field strengths, small electrical discharges occur at the narrowed ends of the droplets and cause concomitant decomposition of the insulation in the region of these discharges. As the insulation material decomposes, additional moisture is permitted to enter and accumulate in the decomposed area of the insulation thereby moving the electrical discharge region further into the insulation to result in a network or pattern of channels or voids throughout the insulation material. This type of insulation deterioration is known as "electrochemical tree formation". Insulation deterioration resulting from electrochemical tree formation causes the breakdown voltage of the insulation in the area of the electrochemical tree to decrease as the size of the electrochemical tree increases.
Over the years considerable effort has been expended in studying the causes and proposing solutions to the problems associated with electrochemical tree formation. For example, U.S. Pat. No. 3,252,834 is representative of an early prior art attempt to treat electrical cables by the introduction of treating compositions into the electrical cable to protect the cable against penetration by moisture and to otherwise prolong the useful life of the cable. The disclosed process involves introducing a treating composition under pressure for the purpose of physically expelling any moisture and air from the interior of the cable and for the purpose of applying a protective coating of the composition on the conductor insulation and interior surfaces of the cable sheathing. The procedure is accomplished by pumping a suitable liquid, such as varnish, silicone oil and solvent, under pressure through a section of the electrical cable. The particular method used permits this to be done even when the cable is in service. Afterwards, the treatment solution is disclosed as being moved from the cable for reuse in treating additional sections of cable. Related to this, U.S. Pat. No. 3,821,640 is directed to an accelerated method of testing the insulation of an insulated conductor for its susceptibility to treeing.
U.S. Pat. No. 4,354,992 is directed to a method and apparatus for fabricating insulated cables, particularly polymeric insulated electrical power cables, which are resistant to the formation and growth of electrical chemical trees.
U.S. Pat. No. 4,145,567, is directed to a solid dielectric cable for high-voltage, alternating current power transmission, which has been fabricated to reduce access of moisture into the cable, particularly to the inner face of the insulation, to such an extent as to make the cable highly resistant to the formation of electrochemical trees in the solid extruded insulation of the cable.
U.S. Pat. No. 4,042,776 is directed to a power cable designed to have a plastic electrical insulation which is partially or wholly composed of a plastic compound which contains an inorganic or organic electrolyte which ionizes when dissolved in water. When the solute dissolves in water, the electric conductivity of the electrolyte is increased in the insulator to render the electric field in a mass of minute voids filled with water relatively small so that the water tree can be prevented.
Recently, electrochemical tree formation and concomitant cable failures have become particularly troublesome in view of the popular use of underground cable systems in residential areas, and continued use of subterranean cables in urban areas. Although various efforts, such as those disclosed above, have been made to alleviate electrochemical tree formation in an attempt to extend the life of such cables and minimize problems which often result because of interruptions in electrical service, none of these efforts have been successful to an acceptable level prior to the invention set forth in U.S. Pat. Nos. 4,372,988 and 4,545,133.
In U.S. Pat. No. 4,372,988, a method and apparatus is disclosed for retarding electrochemical tree-type decomposition of polyolefin electrical insulation by saturating the insulation with a liquid having high resistivity and a low dielectric constant. In particular, this patent provides a method for the in situ repair of an underground high voltage electrical cable composed of a stranded conductor and a polyolefin insulation which involves continuously introducing an electrochemical tree retardant liquid to the conductor so as to cause the liquid to diffuse along between the strands of the conductor and into the insulation layer so as to fill electrochemical trees which may be present in the polyolefin insulation layer. In the case of treating a previously installed cable, this patent discloses that the cable should be dried before introducing the tree retardant liquid into the stranded conductor by heating the cable, for example by passing an electrical current through the conductor of the cable, or by passing a desiccative fluid through the conductor of the cable. The method of the invention set forth in U.S. Pat. No. 4,372,988 is performed by providing a reservoir for supplying electrochemical tree retardant liquid to the conductor at one end of the cable via connection with a flexible tube which is fitted over an end portion of the cable. Although the liquid supply may be accomplished under the force of gravity, a pump may be used to pressurize the liquid and supply the liquid to the input port of a supply valve which, when open, permits the pressurized liquid to pass through the flexible tube and to the conductor at the end of the cable over which the flexible tube is fitted. Thus, the liquid then flows through the inter-strand interstices in the conductor and diffuses through the insulation of the cable so that eventually an entire length of cable and its insulation become saturated with the electrochemical tree retardant liquid.
Related to this, U.S. Pat. No. 4,545,133 is directed to a method and apparatus for retarding electrochemical tree decomposition of electrical insulation by flushing moisture from the insulation of installed power cables with a fluid having a low moisture content, such as nitrogen. By this procedure, the insulation of the power cables is dried, and maintained dried by a continuous flow of dry fluid through the conductor and insulation while the cable is energized. To this end, a dried fluid is disclosed as being introduced into the stranded cable conductor from which it diffuses through the conductor shield and polyolefin insulation. Although the introduction of fluid is disclosed as taking place at the time of installation of the cable, it is also disclosed that the dry fluid may be maintained flowing through the cable while the cable is in use. This is particularly advantageous because the dry fluid will continue to remove the moisture as it reenters the cable insulation structure so as to retard the formation of electrochemical trees. This patent also discloses an apparatus which permits the fluid to be introduced and withdrawn from the cable, particularly while the cable is energized and in use. To this end, the disclosed entry and exit device is installed by stripping the insulation shield from the cable and forming a small hole through exposed insulation near the center of the section of the cable where the insulation shield has been removed. A plastic hose connection is then fastened tightly over the hole using plastic holders, such as plastic ties and rubber washers, in an attempt to minimize leakage of the fluid to the environment, particularly when internal fluid pressures of up to 50 psig are generated while the fluid is being forced through the cable system. It is then recommended that a few layers of electrical insulating tape be placed over the cable insulation in the area where the entry and exit device have been tapped into the cable.
Notwithstanding the effectiveness of the inventions set forth in U.S. Pat. Nos. 4,372,988 and 4,545,133 for treating electrochemical tree deterioration of polyolefin insulation throughout a given length of cable, the methods suffer from certain shortcomings and disadvantages. In this regard, the methods and apparatus of U.S. Pat. Nos. 4,372,988 and 4,545,133 are limited to the entry and exit of the electrochemical tree retardant fluid with respect to a given length of cable in a point-to-point installation between pieces of electrical equipment, such as pad-mount transformers. The main disadvantage with this type of arrangement is that a separate system for introducing and withdrawing the electrochemical tree retardant fluid from the length of cable must be installed at each end of the cable between each pad-mount transformer, switch or other piece of electrical equipment. As a practical matter, such replication of supply reservoirs and entry and exhaust ports are extremely impractical and expensive for use in large-scale cable systems, such as those which would be used for a residential area or other community.
Similarly, although the method for flushing moisture from the insulation of installed power cables with a fluid having a low moisture content, as disclosed in U.S. Pat. No. 4,545,133, is somewhat effective for its intended purposes, the disclosed method and apparatus likewise suffers from disadvantages. In particular, the apparatus disclosed by U.S. Pat. No. 4,545,133 to permit the fluid to enter and exit from the cable has a structure which is particularly difficult to use and is more prone to permit leakage of the fluid from the system than the apparatus disclosed in the present invention.
The method and apparatus of the present invention remedy such shortcomings of U.S. Pat. No. 4,372,988 by providing means for reducing the number of supply reservoirs and related apparatus, by providing an apparatus for connecting lengths of cable to electrical equipment, such as pad-mount transformers, in such a way as to permit the current to pass between the lengths of cables through the piece of electrical equipment while causing the electrochemical tree retardant fluid to by-pass the piece of electrical equipment and pass from one length of cable to another length of cable attached to the equipment in an uninterrupted fashion throughout a large-scale cable system including numerous pieces of electrical equipment interposed between lengths of cable. Thus, the present invention is advantageous in that it permits the treatment of a plurality of lengths of cable connecting various pieces of electrical equipment while requiring only one reservoir for the electrochemical tree retardant fluid in contrast to the system disclosed by U.S. Pat. No. 4,372,988 which requires a liquid supply reservoir and entry and exit ports for each length of cable between pieces of electrical equipment.
Moreover, the method and apparatus of the present invention remedy the previously discussed disadvantages of the entry and exit device disclosed by U.S. Pat. No. 4,545,133 to permit the introduction of fluid into the interstitial spaces of the cable. In contrast to the cut-and-paste type of procedure disclosed by U.S. Pat. No. 4,545,133, a uniquely designed T-shaped fitting has been developed which can be easily inserted between two lengths of cable in a relatively simple and straightforward manner using conventional pieces of electrical equipment to secure the lengths of the cable and the fitting together in a manner which essentially prevents any leakage from occurring.
As will be better understood from the description of the invention presented hereinafter, the present invention is directed to a method and apparatus for treating electrical cables with fluid effective to flush or dry the cable, as well as to retard electrical tree formation, in a relatively simple and straightforward manner using apparatus and devices which permit the introduction of the appropriate fluid continuously through the cable system over a length of cable heretofore not thought possible using novel and unique devices and fittings which have been improved over what has been taught or suggested by the prior art to minimize leakage of the fluid into the environment surrounding the cable system.